Application of soft lithography and micro fabrication on neurobiology

2
Application of Soft Lithography and
Micro-Fabrication on Neurobiology
Gao Kan, Chen Haifeng, Liu Bing-Fang and Xu Qun-Yuan
Beijing Institute for Neuroscience Beijing Center of Neural Regeneration and
Repairing Key Laboratory for Neurodegenerative Diseases of
The Ministry Education Capital Medical University Beijing,
China
1. Introduction
Soft lithography should be regarded as a complement to common lithography, providing a
low-expertise route toward micro/nanofabrication and playing an important role in
microfluidics (YN Xia & Whitesides, 1998). The resolution ranges from 5 to 100 nanometer
(Pilnam Kim, et al. 2008). Patterns generated by the soft lithography are transfered
repeatedly to the soft flexible materials, and then are printed on the medium substrates. In
this field, micro contact printing (μCP) is the most widely used technique, especially in
bioscience research. Combined with microfluidic patterns technology, several kinds of the
extracellular matrix proteins like polymers can be printed to make cells grow according to
the designed patterns (Tai Hyun Park, et al. 2003). The cell growth, differentiation in vitro
can be regulated in the respect of spatial structure of extracellular matrix(Y. Nam, et al.
2004). So the morphology of neural cells and the influence of spatial structure can be
investigated on the micron or even nano-scale level substrates. The closed loop of neural
cells can be constructed in order to simulate the complex neural network in vivo. Finally,
communication with the specific environment in vitro will be achieved by multi-electrode
arrays (MEA). Our previous work used μCP technique can build more solid patterns. By
comparing three different extracellular matrixes, PEI can obtain much better results, which
adhering more neural cells to form reliable design. However, it is not perfect for the specific
neural network construction and the patterned neural cell culture on MEA. In subsequent
research, we improved the parameters of the template and achieved a big progress on
microfluidic patterning technique to microfabricate patterns. Patterns of biomaterials were
constructed with the help of the advanced soft lithography to do the primary cell culture,
such as dopaminergic neurons in the substantial nigra and GABAergic neurons in the
striatum. Finally, the biocompatibility of MEA was validated initially by primary striatal
neuronal culture. Meanwhile, new strategy of structural microfabrication on MEA surface
was further explored.
2. Soft lithography materials
Why soft lithography is called "soft”? One of the reasons is the use of soft organic materials
such as polydimethyl siloxane(PDMS), polymethyl methacrylate (PMMA), and
polycarbonate (PC). PDMS is most widely used in bioscience research, because it has good
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18 Advances in Unconventional Lithography
biocompatibility, chemical stability, optical transparency, air permeability, elasticity.
Moreover, the polymer precursors can be aggregated into a mold by UV radiation. PDMS
polymerization is shown in Figure 1.
Fig. 1. PDMS polymerization
To cure the PDMS prepolymer in general, a mixture of silicon elastomer and a curing agent
(10:1, Sylgard 184 silicone elastomer kit, Dow Corning Corp.) is poured onto the master and
placed at 70-80°C for 1 h. The character of the PDMS is closely related to the mixture ratio,
curing temperature, and vacuum. Silicon, quartz or glass, and some photoresist are the most
common materials to fabricate the masters by standard lithography, transferring the
patterns to the PDMS stamp.
3. Soft lithography fabrication methods and it’s application in patterning
Various soft lithographic technologies have been applied to fabricate high-quality
microstructures and nanostructures including micro contact printing (μCP), replica molding
(REM), microtransfer molding (μTM), micromolding in capillaries (MIMIC), and solvent-
assisted micromolding (SAMIM). Here, three soft lithographic methods are introduced to
fabricate micropatterns onto a surface or MEA: μCP, microfluidic patterning technique and
microstencil. The former two can be achieved using the same PDMS stamps and molds. A
novel technology was applied to get the high depth-to-width ratio silicon-based mold to
fabricate the topographic PDMS microstencil with microfluidic channel (Y.Nam, et al, 2006).
Finally, microchannels on MEA with polyimide (PI) guiding the cell growing were also
introduced.
3.1 Microcontact printing (μCP)
μCP is a direct method for pattern transfer, generating a non-structured, chemically
modified surface. The process of μCP is shown in figure 2. Photolithography was used for
the fabrication of silicon-based masters in preparing PDMS stamps. Multi-layer molds were
made of thick photoresist like SU-8 on silicon or glass wafers by standard lithography
techniques. It was subsequently placed at least 30min in an oven at 160°C to make the
photoresist adhere to the substrate closely. Release agent DC20 or OTS were always spin-
coated and drying on the master before pouring PDMS. Liquid PDMS (Sylgard-184 from
Dow Corning) was poured onto the mold and clamped by the foil, so that the shape of the
mold microstructure was transferred to PDMS membrane. It was subsequently placed at
least 2 h in an oven at 80°C. The molded PDMS slab was then peeled off and placed onto a
glass slide for handling. After curing, PDMS stamps are soaked in a protein “ink”, such as
PEI, PLL, or LN. 20 minutes later, the “ink” was blew off by using nitrogen gas. Then the
raised regions were brought into conformal contact with a substrate in order to print the ink
onto the substrate surface. The material of interest was transferred from the PDMS stamp
onto the substrate surface. The microscopy of the PDMS stamps were shown in figure 3.
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Application of Soft Lithography and Micro-Fabrication on Neurobiology 19
PDMS stamp
Drip into “ink”
Dried in a
gentle nitrogen
stream
Substrate
Pretreatment with
Piranha solution
Printing
Some Pressure
was added
Protein Patterns
Fig. 2. Schematics of the processes of μCP
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20 Advances in Unconventional Lithography
Fig. 3. Microscopy of the PDMS stamps
The bare areas of substrate surface that the PDMS stamp has not touched can be exposed to
another coating material. μCP provides the patterning of self-assembled monolayers(SAMs)
of alkanethiols on gold, and the resulting control over the adsorption of adhesive proteins
facilitates the patterning of cells on substrates.
μCP enables easy stamp replication, fast printing using parallelization, and low-cost batch
production. A conformal contact between the stamp and the surface of the substrate is the
key to its success. The polymer stamps also minimize the problems of sample carry-over
and cross contamination. Printing has the advantage of simplicity and convenience: Once
the stamp is available, multiple copies of the pattern can be produced using straightforward
experimental techniques. Printing is an additive process; the waste of material is minimized.
Printing also has the potential to be used for patterning large areas.
However, μCP has some limitations that are mainly caused by the use of a soft polymer
stamp. The swelling of a stamp during inking often results in an increase in the pattern size
by diffusion of the excessive printed molecules on the substrate.
3.2 Microfluidic patterning using microchannels
The difference between μCP and microfluidic patterning is that PDMS stamps are soaked in
the “ink” in the former usage, but the stamps contact the substrate forming microchannels
delivering the materials for cell adhesion or cell suspension to the desired area in the latter
usage because of the elastic nature and hydrophobicity of PDMS. The substrate tilted 45
degree, drop of liquids were injected to the PDMS microchannels by the pipette. Then the
substrate was put on the test tube rack vertically for 25 to 30 min. Patterns were formed of
after the liquid dried.
While this method has been used primarily for surface attachment of cells, it may be
possible to adapt this method to three-dimensional tissue constructs. In many cases three
dimensional tissue constructs promote cellular differentiation and more authentic cellular
morphology and metabolism.
3.3 Microstencil on MEA
The former two methods enable patterning adhesion molecules and guiding cultured
cells grow physically. But the cells’ communication and interactions in co-cultures are
difficult to be detected, which is important to research the function of the cells network.
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Application of Soft Lithography and Micro-Fabrication on Neurobiology 21
MEA was a cell-based biosensor for extracellular electrophysiological investigations of
neuronal networks. PDMS microstencil was designed to pattern adhesion molecules at
the surface of MEA guiding cultured cells grow along the patterns. PDMS microstencil
mold was fabricated by a complex photomask aligning method, shown in figure 4.
thermal oxidation
SiO2
a Si
Spin-coat photoresist
b Si
HF etching
c Si
Spin-coat photoresist
Si
d
DRIE
e Si
DRIE
f Si
Fig. 4. Schematics of the fabrication of microstencil mold
525μm silicon was thermal oxidated with 4000 Ǻ SiO2 (see Fig. 4a). The substrate was ﬁrst
coated with a thin photoresist (AZ AZ9912) for 30 s at 3000 rpm (see Fig. 4b). 8 × 8 SiO2
arrays were fabricated by wet etching as the RIE mask which was the same as the MEA
structure (see Fig. 4c). Photoresist was spin-coated to the silicon with SiO2 mask again (see
Fig. 4d) and selectively exposed to UV under a chromium photomask. The silicon was
etched 30μm by deep reactive ion etching (DRIE) with photoresist in order to construct the
microchannel , (see Fig. 5a). Then photoresist was removed by ultrasonication in acetone.
The silicon was selectively etched 70μm by DRIE with SiO2 resist, forming the topographic
PDMS microstencil mold (see Fig. 5b). The high depth-to-width ratio silicon-based mold
was designed to penetrate through the PDMS membrane on the MEA to exposure the
electrodes and form the microchannel between the electrodes so that the MEA could also
work.
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22 Advances in Unconventional Lithography
(a) (b)
Fig. 5. SEM images of PDMS microstencil mold
The mixture of PDMS prepolymer and curing agent was spin-coated on the mold for 40 s at
4000 rpm. The coated mold was cured for 2 hours at 110◦C in a convection oven. The fully-
cured PDMS-coated mold was soaked in an acetone ultra-sonication bath until the PDMS
layer released from the mold. The detached microstencil was rinsed with IPA and DI water.
The upside and downside of PDMS microstencil with microholes and microchannels SEM
images was shown in figure 6.
Fig. 6. SEM images of PDMS microstencil
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Application of Soft Lithography and Micro-Fabrication on Neurobiology 23
However, PDMS microstencil is difficult to practise because it is hard to align with MEA
and the silicon mold is easy to fracture when lifting off.
3.4 MEA with microchannels for patterning
Planar MEA are developed to study electrogenic tissues such as dissociated neuronal
cultures (Hiroaki Oka，et al. 1999). They have been widely used with dissociated cultures
for a variety of neuroscience investigation including learning and memory and cell-based
biosensors for the detection of neurotoxins (Conrad D. James, et al. 2004). But the neurons
grow disorderly and cannot form a network so that the function is not the same as the cells
in vitro. Combining the patterning technology and MEA forming neuronal networks is the
efficient method to research the neurobiology.
μCP, microfluidic patterning technique and microstencil are difficult to operate because the
space is too small and the PDMS stamp or stencil can hardly align with MEA. The best and
easiest way to forming neuronal networks is to fabricate the microchannels on MEA with
polyimide (PI) guiding the cell growing.
MEA were fabricated using a conventional semiconductor process (Guangxin Xiang, et al.
2007). After cleaning the polished quartz glass wafer, the conductive layer of Au/Ti ﬁlm
(Au 3000 Ǻ and Ti 700 Ǻ) was sputtered. 8 × 8 electrode arrays were left with the photomask
protection by standard photolithography. Then, a combination of SiO2/Si3N4/SiO2 (3000 Ǻ
/4000 Ǻ /3000 Ǻ) passivation layers was deposited onto the substrate using plasma
enhanced chemical vapor deposition (PECVD), and the insulating layers on the electrodes
and the bonding-pads were removed by inductively coupled plasma (ICP) (see Fig. 7a).
Finally, Negative photosensitive polyimide (AP2210B, Fujifilm Electronic Materials Inc) was
spin-coated to form microchannels having a thickness of 3~4μm and photo-etched by the
standard procedure to expose the microelectrodes and the terminals (see Fig. 7b).
(a) (b)
Fig. 7. Microscopy of the MEA with PI microchannels
GABAergic neurons in the striatum and PC12 cells were cultured on MEA with PI
microchannels which were coated with poly-l-lysine (PLL) to promote cell adhesion, (see
Fig. 8a, 8b). PI microchannels could be seen between the electrodes and the neural cell can
grow along the microchannels. However the nerve cell synapse could not formed along the
microchannels. Because the depth of microchannels could not match the neurons and the
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24 Advances in Unconventional Lithography
PLL could not be guarantee to coat the microchannels effectively after days. There is still a
lot of work to do to construct the neuronal networks on MEA to study the cells function as
in vitro.
(a) (b)
Fig. 8. Microscopy of the cells cultured on MEA with PI microchannels (a) GABAergic
neurons in the striatum cultured at 3 days (b) PC 12 cells cultured at 6 days
4. Construction of neural network by applying soft lithography
A man-made neural network on electrode can be applied to do basic research of
neuroscience, be a kind of biosensor for drug discovery [9] and even be implanted into brain
to establish artificial connections that could form the basis of a neural prosthesis[10]. These
fields have caused much attention in the world [11]. In the past, there are three strategies to
realize a simplified neural network in vitro, such as mechanical fixation-applying spatial
restrictions[12,13], physical modification of surface roughness and surface
topography[14,15], chemical polymer microfabrication on surface—using soft lithography.
Soft lithography is created by Whitesides in 1993[16]. It is used to create chemical structures
on surfaces, including µCP, µFN and other downstream techniques. These microfabrication
techniques that control both the size and shape of the cell anchored to a particular surface
are extremely useful for understanding the inﬂuence of the cell–material interface on the
behavior of cells [17, 18].The adhesion and survival of neural cells should be considered
firstly for the patterned neural cell culture in vitro. So the selection of appropriate cell-
attracting substances is an important step for pattern design in micro-contact printing. Ideal
substances encourage good cell–substrate interactions, constantly stimulate the cells by
substrate-bound chemical, biological, electrical or mechanical signals [19, 20] and even
regulate neuritis growth on designed patterns. The most commonly used coating reagents to
promote cell adhesion are extracellular matrix (ECM) proteins like laminin (LN), positively
charged polymers such as poly-l-lysine (PLL) and synthetic amide-linkage-free compounds
such as polyethylenimine (PEI). Therefore, the characteristics of three different substrates,
PEI, PLL, LN were compared by the primary neuron culture in our previous work. The PEI
characterized with strong positive surface charges was validated to fabricate more
continuous and integrated micro-contact printing neural patterns under serum-free culture
conditions than PLL, LN[21].
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Application of Soft Lithography and Micro-Fabrication on Neurobiology 25
For the functional neural network construction on MEA, an inevitable question that should
be addressed finally is how to realize accurate opposite between neurons and the electrode
under neurons. We assume firstly microfluidic technique may have more advantages than
μCP. In subsequent research, we achieved satisfied patterns by microfluidic technique for
further research with the help of the progress on parameters of template. Specific neural
network were constructed by applying advanced soft lithography above to do the primary
cell culture, such as dopaminergic neurons in the substantial nigra and GABAergic neurons
in the striatum. The conditions of neuronal adhesion on different patterns (grids and lines)
were also observed using several techniques, including atomic force microscopy,
immunohistochemistry, transmission electron microscope and scanning electron
microscope.
4.1 Neural network with rat fetal hippocampal cells by μCP patterns
In previous study, we examined the ability of another positively charged polymer,
polyethyleneimine (PEI), to promote neuronal adhesion, growth and the formation of a
functional neuronal network in vitro. PEI, PLL and LN were used to produce grid-shape
patterns on glass coverslips by μCP. Post-mitotic neurons from the rat fetal hippocampus
were cultured on the different polymers and the viability and morphology of these neurons
under serum-free culture conditions were observed
4.1.1 Cells adhesion
The number of cells that adhere to the different substrates after 24 h in culture is shown in
Fig. 9. The adhesive effects were evaluated by calculating the ratio of cell numbers that
adhere to the grid-like patterns divided by the total area of printed polymer. We found that
the positively charged polymers (PLL and PEI) had a signiﬁcantly higher level of cell
attachment than LN (p < 0.05)[13].
Fig. 9. The adhesive effects were test by analysis of the number of neurons on the area
(mm2) of LN, PEI and PLL grid patterns after 24 h in culture. The asterisks indicated
neurons on PEI and PLL patterns had signiﬁcantly higher lever than on LN patterns, n = 12,
p < 0.05.
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26 Advances in Unconventional Lithography
4.1.2 Cells viability
In Fig.10, We show that neurons cultured on the PEI- and PLL-coated surfaces adhered to
and extended neurites along the grid-shape patterns, whereas neurons cultured on the LN-
coated coverslips clustered into clumps of cells. In addition, we found that the neurons on
the PEI and PLL-coated grids survived for more than 2 weeks in serum-free conditions,
whereas most neurons cultured on the LN-coated grids died after 1 week[13].
Fig. 10. Images obtained using the phase contrast microscope, showing cells cultured on PEI,
PLL and LN polymeric ﬁlms at different time points. (a, d, and g) Representative images of
neurons cultured on PEI grid patterns at 24 h, 7 days, and 14 days, respectively, show that
neurons adhere and grow accurately along the PEI grids at differently time points. (b, e, and
h) Representative images of neurons cultured on PLL grid patterns at 24 h, 7 days, and 14
days, respectively. (c, f, and i) Representative images of neurons cultured on LN grid
patterns at 24 h, 7 days, and 14 days, respectively. The images (c and f) show that the
neurons on the LNgrid patterns often accumulate at the cross points of the grids. The image
(i) shows that most neurons disappear after 14 days in culture and only small areas of the
neural cells exist. (a–c) Bar = 100 μm; (d–i) bar = 50 μm.
4.2 Specific neural network with two relative neurons, such as dopaminergic neurons
and GABAergic neurons
There is a closely relationship in the respect of function and structure between dopaminergic
neurons in the substantial nigra and GABAergic neurons in the striatum. As well-known,
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Application of Soft Lithography and Micro-Fabrication on Neurobiology 27
Parkinson's disease is due to the loss and injuries of dopaminergic neuron in substantia
nigra which cause a decrease in nerve fibers projected to the new striatum. Finally,
reasonable synaptic connections and neural network can not be established. Therefore, it is
expected to establish cell models to investigate the relationship between these two kinds of
coherent neurons and construct an artificial neural network in vitro by the application of
soft lithography. In present work, specific neural network with dopaminergic neurons and
GABAergic neurons co-culture was established by μCP PEI grid patterns. Meanwhile, PEI
was validated again to fabricate more continuous and integrated neural patterns by using
μCP and microfluidic technique both.
4.2.1 Neural network by μCP
Different kinds of neural network by μCP were established with neuron from the
striatum, dopaminergic neurons from the substantial nigra and both of them co-culture.
The conditions of neuronal adhesion on different pattern figures were observed using
several techniques, including immunocytochemical staining, transmission electron
microscope and scanning electron microscope. Using immunocytochemical staining,
transmission electron microscope, we identified the types of neural cells and observed
some neurosynapse-like structures near the neuronal soma on PEI-coated coverslips.
These ﬁndings indicate that PEI is a suitable surface for establishing a functional neuronal
network in vitro.
4.2.1.1 Investigation of neural cell types and neurite elongation along the grid-like
patterns by immunocytochemical staining and SEM
PEI, PLL and LN were used to produce grid-shape patterns on glass coverslips by micro-
contact printing. GABAergic neurons and medium spiny neuron from the rat striatum,
dopaminergic neurons from the rat substantial nigra and both of them co-culture were
researched separately on the different polymers coated surface. The viability and
morphology of these neurons under serum-free culture conditions were observed using
ﬂuorescent microscopy in Fig. 11, Fig. 13, Fig. 14. After 7 days in culture, we found that
the neural cell bodies on the PEI patterns were located mostly at the cross-points of the
grid, whereas neurites extended along the line of the grid-like patterns. More continuous
and integrated neural network was achieved finally. On the PLL-coated coverslips, the
neural patterns appeared to be integrated. But several cells clustered at the cross-points of
the grid disappeared gradually after the media was replaced. In contrast, cells cultured on
the LN-coated grids were generally clustered into clumps and cannot form satisfied
patterns. Different sizes of PEI pattern were produced by microcontact printing. In Fig. 12,
compared with 50μm, 100μm, 200μm pattern sizes, we found few difference early.
After 7 days or 14 days culture, most neural cells on 200μm size grew well and were
seldom found to overlap each other, unlike those on 50μm size clustered into clumps at
the cross-points of grid and disappeared gradually. Identified with immunocytochemical
staining, we found that neural cells from the rat substantial nigra were TH positive,
synaptic vesicle protein were synaptophysin positive in Fig. 13 and cells from the rat
striatum were GABA positive or DARPP-32 positive in Fig. 11, Fig. 12, Fig. 14. SEM show
neurons outgrowth on PEI-coated patterns and validate the findings by
immunocytochemical staining in Fig. 15, Fig. 16.
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28 Advances in Unconventional Lithography
(a) on LN grid patterns (b) on PEI 100μm grid patterns (c) on PLL 100μm grid patterns bar=50μm
Fig. 11. Immunofluorescent image of anti-GABA (green fluorescence) +anti-MAP2 (red
fluorescence) labelled striatal neurons cultured for 7 days on different substrates, the nuclei
of neurons were stained with Hochest x400
(a)on 50μm patterns (b) on 100μm patterns (c) on 200 μm patterns bar=50μm
Fig. 12. Immunofluorescent image of anti-GABA(green fluorescence)+anti-MAP2(red
fluorescence) labeled striatal neurons cultured for 7 days on different sizes of PEI patterns,
the nuclei of neurons were stained with Hochest x400
TH+Syn
(a) on LN grid patterns (b) on PEI 100μm grid patterns (c) on LN+PEI bar=20μm
Fig. 13. Immunofluorescent image of anti-TH(red fluorescence) labeled dopaminergic
neurons from the substantial nigra and anti-synaptophysin(green fluorescence) labeled
synaptic vesicle protein cultured for 7 days on different substrates. x200
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Application of Soft Lithography and Micro-Fabrication on Neurobiology 29
(a–c) co-culture neurons growing on LN grid patterns. (a) Immunostaining with anti-TH (red
ﬂuorescence) labelled dopaminergic neurons. (b) Immunostaining with anti-DARPP-32 (green
ﬂuorescence) labeled medium spiny neuron from the striatum(arrow). (c) Merged image of (a) and (b),
showing TH positive neurons on the LN-coated patterns actually adhere to the cluster formed by
medium spiny neuron. (d–f) co-culture neurons growing on PEI grid patterns. (d) Immunostaining with
anti-TH (red ﬂuorescence) labelled dopaminergic neurons(arrows). (e) Immunostaining with anti-
DARPP-32 (green ﬂuorescence) labeled medium spiny neuron from the striatum(arrow). (f) Merged
image of (d) and (e) showing that DARPP32 positive neurons cultured on the PEI-coated patterns form
a continuous and integrated neural network, and two TH positive neurons adhere to the cross-points of
grid. (g–i) co-culture neurons growing on PLL grid patterns. (g) Immunostaining with anti-TH (red
ﬂuorescence) labeled dopaminergic neurons(arrow). (h) Immunostaining with anti-DARPP-32 (green
ﬂuorescence) labeled medium spiny neuron from the striatum(arrows). (i) Merged image of (g) and
(h)showing only two DARPP32 positive neurons adhere to the cross-points of grid. TH positive neurons
were not restricted by the grid pattern. bar=100μm
Fig. 14. Immunofluorescent image of anti-TH labelled dopaminergic neurons from the
substantial nigra and anti-DARPP32 labeled medium spiny neuron from the striatum co-
culture for 7days on different substrates, the nuclei of neurons were stained with Hochest
x200
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30 Advances in Unconventional Lithography
Fig. 15. SEM show neurons outgrowth on PEI-coated patterns. Somata of neuons located on
the cross point and neurites extend along the lines
Fig. 16. SEM show that Line of grid were occupied by abound neurites.
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Application of Soft Lithography and Micro-Fabrication on Neurobiology 31
4.2.1.2 Examination of synaptic formations by immunocytochemical staining and TEM
To further understand functional activities of neurons on the grid pattern, we observed the
microstructure of the neuronal cell body, neuritis extension and processes of the synapse
formation by using laser confocal microscope after 7 days in culture. It is found that several
neural cell bodies aggregated on the cross point of the grid pattern (Fig.17.b), which neurites
extended along the lines clearly by cytoskeletal proteins MAP2 and Synaptophysin double
immunocytochemical staining of patterned neurons on PEI(Fig.17.a). Some processes can
even gather into a bundle, span the distance between two cross-points and make connection
with another neuron. Synaptophysin was a kind of granular protein scattered around the
cell bodies and neurites, The neurites from neuron at the cross-point seemed to
communicate with those at the lines which express a large number of synaptic vesicle
protein(Fig.17.c).
MAP2+Syn
(a) neurites extended along the lines and synaptic vesicle protein around
(b) neural cell bodies at the cross-point and synaptic vesicle protein scattering around them
(c) synapse connection between two neurons at the grid pattern bar=25μm
Fig. 17. Immunofluorescent image of anti-MAP2(red fluorescence) labeled neurons and anti-
synaptophysin(green fluorescence) labeled synaptic vesicle protein cultured for 7 days on
PEI patterns x400
We chose PEI group which can construct more clearly neural network and continued to
observe intercellular ultrastructure under transmission electron microscope (Figure18). Two
periphery of neurites thicken show high electron density to form a synaptic contact (Red
border). Width of synaptic cleft was measured to 30 ~ 50nm. Figure18(a) shows clear
synaptic vesicles. Figure18(b) shows the synaptic cleft is relatively narrow and suspected to
be electrical synapse structure due to double-membrane structure adjacent closely.
4.2.2 Neural network by microfluidic technique
In this experiment,, we made a big progress on microfluidic technique by re-designing the
parameters and enhancing the photoresist coating thickness of the Cr template. After 7 days
in culture, poly-l-lysine and laminin+polyethyleneimine were found to be formed more
complete and clearer flow patterns by the application of microfluidic technique. On LN
group, neurons were easy to cluster into clumps when channel width was 150μm,almost
overshadowed flow pattern itself; On PEI group, even though the flow patterns are more
complete, the neurites extend short and cannot constitute a connection between some cells
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32 Advances in Unconventional Lithography
(a)chemical synapse structure x40000 bar=0.5μm (b)electrical synapse structure x60000 bar=0.2μm
Fig. 18. Under TEM, synapse like structure between neurons from the striatum and the
substantial nigra on PEI patterns
(a) LN group, width of lines 150μm, (b) PLL group, width of lines 150μm (c) PEI group, width of lines
200μm , (d) LN+PEI group, width of lines 300μm bar=200μm
Fig. 19. Immunofluorescent image of anti-GABA (green fluorescence) +anti-MAP2 (red
fluorescence) labelled striatal neurons cultured for 7 days on different substrates by
microfluidic technique x100
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Application of Soft Lithography and Micro-Fabrication on Neurobiology 33
sufficiently. When the channel spacing of 300μm, LN + PEI group of cells in the liquid
injection port at the distribution, flow pattern clear, but the group off between individual
cells, so that flow interruption. Identified with immunocytochemical staining, we found that
most of the MAP-2 positive neural cells from the striatum cultured on the flow pattern were
also labeled with GABA, as shown in Figure 19.
Observing the fine structure of the intercellular on PLL group under scanning electron
microscopic in Fig. 20, most of the neural cell bodies were found adhere to the flow pattern
and majority of neurites are constrained within the width of the channel to grow following
the orientation of flow channels. It is still visible that a few neuronal cell bodies deviate from
the flow patterns slightly or adhere on the blank between two lines by local amplification.
By observing the cross-linked region off low channel, we can see two endings of neurites
seem to have varicose, swelling structure liking synapse.
(a)x250 bar=100μm (b)x1000 bar=10μm (c)x10000 bar=1μm
Fig. 20. Neurons from the striatum and the substantial nigra on PLL microfluidic patterns
under SEM.
5. Conclusion
In summary, this study based on previous work improve the microfluidic technique,
evaluate the influence of two different soft lithography, the micro-contact printing and
microfluidic technique on various interface materials for the construction of neural network.
Our future work can be divided into two levels. On the one hand continue to look for the
intersection of microelectrode array and the micro-fabrication technology, trying to make
cell grow in accordance with patterns of MEA electrode nodes. On the other hand, need to
further improve the characteristics of electrode materials, enhance biocompatibility under
the premise of improving signal-to-noise ratio and without adding resistance, do good to
the survival of neurons and neurite extension.
The application of the micro-fabrication on microelectrode array may open up a broader
platform of the technique for neurochip research and provide new ideas for the treatment of
various injuries in the central nervous system. Ultimately, the combination both can achieve
position fixing between neurons and electrodes precisely, to make the system as a real
sensor, be able to accept electrical stimulation or chemical stimulation and record their
signals, to analysis the transfer process of neural network information, and apply to drug
screening of related diseases.
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Pilnam Kim, Kahp Yang Suh, et al. (2008). Soft Lithography for Microfluidics: a Review.
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www.intechopen.com
Advances in Unconventional Lithography
Edited by Dr. Gorgi Kostovski
ISBN 978-953-307-607-2
Hard cover, 186 pages
Publisher InTech
Published online 09, November, 2011
Published in print edition November, 2011
The term Lithography encompasses a range of contemporary technologies for micro and nano scale
fabrication. Originally driven by the evolution of the semiconductor industry, lithography has grown from its
optical origins to demonstrate increasingly fine resolution and to permeate fields as diverse as photonics and
biology. Today, greater flexibility and affordability are demanded from lithography more than ever before.
Diverse needs across many disciplines have produced a multitude of innovative new lithography techniques.
This book, which is the final instalment in a series of three, provides a compelling overview of some of the
recent advances in lithography, as recounted by the researchers themselves. Topics discussed include
nanoimprinting for plasmonic biosensing, soft lithography for neurobiology and stem cell differentiation,
colloidal substrates for two-tier self-assembled nanostructures, tuneable diffractive elements using
photochromic polymers, and extreme-UV lithography.
How to reference
In order to correctly reference this scholarly work, feel free to copy and paste the following:
Gao Kan, Chen Haifeng, Liu Bing-Fang and Xu Qun-Yuan (2011). Application of Soft Lithography and Micro-
Fabrication on Neurobiology, Advances in Unconventional Lithography, Dr. Gorgi Kostovski (Ed.), ISBN: 978-
953-307-607-2, InTech, Available from: http://www.intechopen.com/books/advances-in-unconventional-
lithography/application-of-soft-lithography-and-micro-fabrication-on-neurobiology
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